Automatic control systems



28, 1965 H. O.WAL.KER

AUTOMATIC CONTROL SYSTEMS 2 Sheets-Sheet 1 Filed Feb. 4, 1965 T/ME Inventor HQIAEY Z aswa/ MQ/kcr y Attorney Dec. 28, 1965 H. o. WALKER 3,226,558

AUTOMATIC CONTROL SYSTEMS Filed Feb. 4, 1963 2 Sheets-Sheet 2 24 22 l6 l6 l4 m 4 mw [9 Q go 2 lnvenlor fi ereri' Oswa la/ WP/Ker i440! Attorney United States Patent 3,226,558 AUTOMATIC CGNTROL SYSTEMS Herbert Oswald Walker, West Hartlepool, England, as-

signor to Richardsons, Westgarth & Co. Limited, Northumberland, England Filed Feb. 4, 1963, Ser. No. 255,774 Claims priority, application Great Britain, Feb. 5, 1962, 4,340/62 6 Claims. (Cl. 290-40) This invention relates to automatic control systems for controlling the operation of rotary machines such as steam turbines. The invention is particularly applicable to the control of these machines during their starting-up period.

The starting-up of large steam turbines is frequently controlled through a control system which regulates the operation of the turbine up to the point where the normal running control gear takes over, the regulation being achieved by using a device which compares the actual speed of the turbine shaft at any instant which the intended speed set by speed-programme apparatus. If for any reason the actual turbine speed departs by more than a limited amount from the intended speed laid down by the programme, some kind of over-ride control can come into action so that the turbine is shut down for correction of the fault which caused the actual speed to depart from the intended speed.

In many instances the departure of the turbine speed from its intended value is due to a temporary factor which does not warrant shutting down the turbine. For example, the speed of the turbine may drop during its starting-up period due to a temporary restriction of the steam supply which is corrected in a relatively short time. A difficulty which arises however if the turbine is kept running under such conditions is that, when the turbine speed lags behind the programme speed to an appreciable extent, the programme must not be allowed to continue as if there had been no such lag in speed, for this would cause a sudden upward surge in the turbine speed on removal or correction of the cause of the speed-lag which could result in serious damage. This in fact is the reason why over-ride controls are provided in the systems used hitherto so that the turbine is shut down on the occurrence of any undesirable departure in speed.

A control system in accordance with the invention therefore includes means which are capable of overcoming the difficulty outlined above, and according to the invention an automatic control system for controlling the operation of a rotary machine comprises speed-sensitive means for detecting non-permissible departures in the speed of the machine from the intended speed, and an adjusting device for adjusting speed-programme apparatus which determines the intended speed so that, on the removal or correction of the factor which caused the actual speed to depart from its intended value, the speed programme recommences at an adjusted intended speed which is not different to a harmful extent from the actual speed of the machine at that instant.

In the case of a turbo-alternator, a similar control system can be used to control the rate of loading of the alternator after it has been synchronised with the electrical network to which it is connected. A control system for this purpose will accordingly have load-sensitive means and load-programme apparatus instead of speed-sensitive 3,226,558 Patented Dec. 28, 1965 means and speed-programme apparatus, the load-sensitive means being arranged to detect non-permissible departures in the load on the machine from the intended load.

In order that the invention may be thoroughly understood, an example of an automatic control system in accordance with it will now be described with reference to the accompanying drawings, in which:

FIGURE 1 is a speed/time graph;

FEGURE 2 is a diagrammatic layout of the control system; and

FIGURE 3 is another speed/time graph.

The line AB in FIGURE 1 represents the intended speed of a steam turbine during its starting-up period, the point B being that at which the normal running control gear takes over. The lines ab on either side of the line AB represent the maximum limits by which the actual speed of the turbine may depart from its intended speed without endangering the machine. In a typical case a deviation of about rpm. from the intended speed is permissible.

If now a fault occurs during the starting-up period at the point C due, for example, to a temporary decrease in the available steam supply, then the turbine speed will fall off along the line CD. Within, say, a few minutes, this fault is rectified at the time T, and the speed of the turbine is now able to increase again. However, the intended speed at the time T laid down by the original programme is that indicated on the line AB at E. Were the operation of the turbine to be governed still by the original programme, its speed would surge as quickly as possible from D to E, with the consequent risk of serious damage being done. This danger is avoided in the control system used hitherto by providing an over-ride control which shuts down the turbine directly its speed crosses either one of the lines ab, but in accordance with the present invention means are provided by which the turbine is kept running and by which the programme is adjusted instead to cater for the interruption which has occurred.

FIGURE 2 illustrates one particular control system by which the speed programme is adjusted to take into account temporary departures of the turbine speed from its intended value. A speed-sensitive device 10 arranged on or near the steam turbine 12 being controlled serves to feed an electrical signal F2 proportional to or dependent on the actual speed of the turbine shaft back to an algebraic summing device 26 such as a summing amplifier which compares the signal with an electrical reference signal F1 received from a program speed signal generator 16 to derive a difference or error signal which is applied over line 28 to the input of a servo amplifier 14. The signal F1 is a variable signal representing the desired or program speed of the turbine shaft at any instant, and provided the relationship between the two signals F1 and F2 is such that the actual speed of the turbine shaft is above the lower line ab on the graph shown in FIGURE 1, the difference or error signal applied to the servo amplifier 14 is below the threshold value necessary to activate the servo and the servo will remain out of action. The program speed signal generator 16 is driven by the combination of a servo amplifier 24 and servometer 22 so as to deliver a program speed signal that increases progressively with time along the curve AB of FIGURES 1 and 3. In the absence of any interruption in the speed I programme, the actual speed of the turbine shaft will 3 approximately follow the line AB shown in FIGURES 1 and 3.

Let us assume that at the point G shown in FIGURE 3 the speed of the turbine shaft is 1,000 r.p.m. A temporary restriction in the available steam supply then occurs inadvertently which causes the turbine speed to drop to 900 rpm, i.e. to the point H on the graph shown in FIGURE 3. The effect of this is that the actual speed signal F2 will immediately drop in relation to the program speed signal F1, and directly the difference between the two signals exceeds an amount corresponding to the distance between the line AB and the lower line ab in FIGURE 3, the input to the servo amplifier 14 exceeds the aforesaid threshold value thereof and the amplifier commences to supply a driving signal F3 to an associated servo motor 18 which serves to drive the program speed signal generator in the opposite direction to the drive imparted by the servomotor 22. The two drives from the two servomotors 22, 18 may be combined by differential gearing (not shown) and thence applied to the program speed signal generator 16 as a single drive input. In this way, the signal F1 representing the program speed of the turbine will be adjusted to a value I which brings the actual turbine speed at the point K back within the permissible deviation from the adjusted programme (see FIGURE 3). As a result, the dilference signal on the input lead 28 falls back below the threshold value of the amplifier 14, the servomotor 18 ceases to drive, and program speed signal generator 16 resumes its normal forward programme but from the adjusted value I. At the point L on the programme, however, the actual turbine speed again lags behind by more than the permissible amount, and the servo amplifier 14 once more brings the servo motor 18 into operation to make a further program speed adjustment. This cyclical process will continue until the steam supply becomes normal again. During this period the line KH representing the actual turbine speed approximately coincides with the line cd representing the maximum permissible deviation from the adjusted program line I M. On resumption of the normal steam supply, the adjusted program speed M will not differ to a harmful extent from the actual speed H of the turbine at that instant. In other words, the turbine will not be required to speed up to a dangerous extent which would have been the case had the programme not been adjusted to compensate for the temporary loss in steam. The programme and the turbine speed now follow the line MN.

As shown in FIGURE 2, the control system includes apparatus 20 of conventional construction to control the steam supply to the turbine 12 in accordance with the signal F1 received from the program speed generator 16 and the signal F2 representing actual turbine speed. The program speed signal generator 16 will generally take the form of a variable ratio transformer with a movable top or wiper driven by the servo motor 22 under the control of the servo amplifier 24.

The specific control system described above is primarily concerned with the control of a steam turbine, but the invention is applicable to the control of other rotary machines such as gas turbines, electric motors and alternators.

I claim:

1. Apparatus for controlling the operation of a steam turbine during start-up and in accordance with a desired program of increasing turbine shaft speed, comprising: an actual speed signal generator sensing actual turbine shaft speed and deriving a first variable signal representative thereof, a program speed signal generator and first drive means therefor, said program speed signal generator deriving a second variable signal that normally increases progressively during turbine start-up and is representative of the desired program speed at each time instant, comparator means receiving said first and second signals as inputs and deriving an error signal representative of the instantaneous difference between actual speed and program speed, a second drive means coupled to said program speed signal generator which second drive means is ordinarily inactive, and servo means responsive to said error signal and applying a driving signal to said second drive means whenever said error signal exceeds a predetermined limit value, said second drive means imparting drive to said program speed signal generator in opposite sense to said first drive means.

2. Apparatus for controlling the operation of a rotary machine during a period of varying output and in accordance with a program of desired change of a selected output parameter, comprising: a signal generator sensing the actual instantaneous value of said output parameter and deriving a first variable signal representative thereof, a program signal generator and first drive means therefor, said program signal generator deriving a second variable signal representative of the desired value of said output parameter at each time instant, comparator means receiving said first and second signals as inputs and deriving an error signal representative of the instantaneous difference between the actual value and the program value of said parameter, a second drive means coupled to said program signal generator which second drive means is ordinarily inactive, and servo means responsive to said error signal and applying a driving signal to said second drive means whenever said error signal exceeds a predetermined limit value, said second drive means imparting drive to said program signal generator in a direction to reduce the difference between said first and second signals.

3. Apparatus for controlling the operation of a rotary machine during start-up and in accordance with a program of desired increase of a selected output parameter, comprising: a signal generator sensing the actual instantaneous value of said output parameter and deriving a first variable signal representative thereof, a program signal generator and first drive means therefor, said program signal generator deriving a second variable signal that normally increases progressively during start-up and is representative of the desired value of said output parameter at each time instant, comparator means receiving said first and second signals as inputs and deriving an error signal representative of the instantaneous difference between the actual value and the program value of said parameter, a second drive means coupled to said program signal generator which second drive means is ordinarily inactive, and servo means responsive to said error signal and applying a driving signal to said second drive means whenever said error signal exceeds a predetermined limit value, said second drive means driving said program signal generator in a direction opposite to said first drive means.

4. Apparatus for controlling the operation of a rotary machine during a period of varying output and in accordance with a program of desired change of a selected output parameter, comprising: a signal generator sensing the actual instantaneous value of said output parameter and deriving a first variable signal representative thereof, a program signal generator deriving a second variable signal representative of the desired value of said output parameter at each time instant, comparator means receiving said first and second signals as inputs and deriving an error signal representative of the instantaneous difference between the actual value and the program value of said parameter, and program-adjusting means coupled to said program signal generator and having said error signal applied thereto as an input said program-adjusting means remaining inactive while said error signal is below a preselected limit value but effecting program-adjusted in said program signal generator in the sense necessary to bring the error signal back below the limit if said limit is exceeded.

5. Apparatus according to claim 2, wherein said signal generators deliver said first and second variable signals to said comparator means as electrical signals, and said first and second drive means comprise servo motors imparting mechanical .drive to said program signal generator.

6. Apparatus according to claim 1, further comprising control means receiving as inputs said first and second variable signals and operative to vary the steam input to the turbine in response thereto.

References Cited by the Examiner UNITED STATES PATENTS Johntz et a1. 13717 Bevins 290403 Carolus et a1. 29052 Bracutt 318398 Guth 29040.2

Chilman 29040 FOREIGN PATENTS Great Britain.

10 ORIS L. RADER, Primary Examiner.

G. SIMMONS, Assistant Examiner. 

1. APPARATUS FOR CONTROLLING THE OPERATING OF A STEAM TURBINE DURING START-UP AND IN ACCORDANCE WITH A DESIRED PROGRAM OF INCREASING TURBINE SHAFT SPEED, COMPRISING: AN ACTUAL SPEED SIGNAL GENERATOR SENSING ACTUAL TURBINE SHAFT SPEED AND DERIVING A FIRST VARIABLE SIGNAL REPRESENTATIVE THEREOF, A PROGRAM SPEED SIGNAL GENERATOR AND FIRST DRIVE MEANS THEREFOR, SAID PROGRAM SPEED SIGNAL GENERATOR DERIVING A SECOND VARIABLE SIGNAL THAT NORMALLY INCREASES PROGRESSIVELY DURING TURBINE START-UP AND IS REPRESENTATIVE OF THE DESIRED PROGRAM SPEED AT EACH TIME INSTANT, COMPARATOR MEANS RECEIVING SAID FIRST AND SECOND SIGNALS AS INPUTS AND DERIVING AN ERROR SIGNAL REPRESENTATIVE OF THE INSTANTENEOUS DIFFERENCE BETWEEN ACTUAL SPEED AND PROGRAM SPEED, A SECOND DRIVE MEANS COUPLED TO SAID PROGRAM SPEED SIGNAL GENERATOR WHICH SECOND DRIVE MEANS IS ORDINARILY INACTIVE, AND SERVO MEANS RESPONSIVE TO SAID ERROR SIGNAL AND APPLYING A DRIVING SIGNAL TO SAID SECOND DRIVE MEANS WHENEVER SAID ERROR SIGNAL EXCEEDS A PREDETERMINED LIMIT VALUE, SAID SECOND DRIVE MEANS IMPARTING DRIVE TO SAID PROGRAM SPEED SIGNAL GENERATOR IN OPPOSITE SENSE TO SAID FIRST DRIVE MEANS. 